Turbocharged and supercharged engines pressurize air entering an engine so that engine power can be increased. The pressurized air provides for an increased cylinder air charge during a cycle of the engine as compared to a naturally aspirated engine. Further, the cylinder fuel charge can be increased as the cylinder air charge is increased to increase the amount of energy produced when the fuel is combusted with the air during a cycle of the cylinder. However, during periods of valve overlap where both intake and exhaust valves of a cylinder are simultaneously open, it is possible for air to pass directly from the engine intake manifold to the engine exhaust manifold without participating in combustion within a cylinder. Air passing directly from the intake manifold to the exhaust manifold without participating in combustion may be referred to as blow-through.
Fresh air or blow-through passing through the intake manifold to the exhaust manifold may have beneficial as well as undesirable characteristics. For example, blow-through can evacuate internal exhaust residuals from engine cylinders so that the fresh charge in the cylinder increases, thereby increasing engine power output. However, blow-through may also upset a delicate balance between oxygen, hydrocarbons, and CO in a catalyst in the engine exhaust path. If blow-through provides excess oxygen to the catalyst, it may be possible for NOx conversion efficiency to decrease.
One way to mitigate engine emissions that may result from blow-through is to account for blow-through gases. In one example, engine air-fuel ratio may be richened so that on average a stoichiometric air-fuel ratio mixture passes through engine cylinders. For example, for a direct injection engine where fuel is injected after exhaust valve closing, fresh air can flow from the intake manifold to the exhaust manifold during intake and exhaust overlap. Further, an increased amount of fuel may be supplied to an engine cylinder so that the cylinder combusts a rich air-fuel mixture. The contents of the cylinder may be subsequently combined with the blow-through air stored in the catalyst to provide near stoichiometric ratios of gases to the catalyst so that the catalyst can efficiently oxidize and reduce undesirable exhaust gas constituents. However, it is difficult to keep the catalyst balanced and adjust air-fuel ratio of a cylinder without knowing the amount of blow-through for speed-density control systems.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for accounting for cylinder blow-through of an engine, comprising: adjusting an engine actuator controlling supply of a constituent for combustion to a cylinder of the engine in response to a difference between a total cylinder air mass flow curve and a volumetric efficiency curve.
A mass of oxygen that reaches an exhaust after treatment device resulting from cylinder blow-through may be determined from two curves that characterize engine breathing. In particular, cylinder blow-through may be determined as a difference between a first curve that represents volumetric efficiency for a theoretical maximum cylinder air charge and second curve that represents total air flow through the cylinder. Both curves may be described according to slopes of lines so that cylinder blow-through may be determined without having to perform a significant number of calculations and without having to determine a cylinder air-fuel ratio.
In another example, the amount of blow-through can be adjusted to increase engine output and to provide a desired amount of oxygen to the exhaust system to promote regeneration of an exhaust gas emissions device. For example, blow-through may be used during regeneration of a particulate filter to oxidize stored carbonaceous soot. The amount of blow though can affect the temperature rise of the oxidizing carbonaceous shoot during regeneration. Therefore, it may be desirable to determine the amount of blow though so that a desired level of blow-though may be provided to the emissions device without supplying excess blow-through.
The present description may provide several advantages. In particular, the approach can reduce vehicle emissions by providing an accurate blow-through estimate so that an amount of air reaching an exhaust gas after treatment device may be determined. Further, an engine actuator may be adjusted so as to control the amount of blow-through supplied to the exhaust gas after treatment device. Further still, the method provides for determining an engine operating condition where blow-through begins so that blow-though can be controlled during conditions where increased engine power is desired or when exhaust gas constituents entering a catalyst may be balanced so as to improve conversion efficiency of engine exhaust emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.